Conventionally, a piezoelectric transformer has been used in a power supply of a variety of small-sized electronic apparatus. Typically, Rosen type and modified Rosen type transformers are known.
An example of the Rosen-type piezoelectric transformer comprises a rectangular piezoelectric ceramic plate, a pair of input-side electrodes formed on top and bottom surfaces thereof over an approximate half in its longitudinal direction, and an output-side electrode formed on an end surface of the other half where the above-mentioned pair of input-side electrodes are not formed. One of lead wires respectively connected to the pair of input-side electrodes serves as an input terminal of an input portion. One of lead wires connected to the output-side electrode serves as an output electrode kept at a high potential. The other lead wire connected to the input-side electrode serves as an input/output common terminal.
On the other hand, an example of the modified Rosen-type piezoelectric transformer comprises a layered structure formed by alternately stacking inner electrode films and piezoelectric ceramics. An approximate half of the layered structure in its longitudinal direction is used as an input portion while the other half is used as an output portion. In the input portion, every adjacent ones of the inner electrodes alternately stacked with the piezoelectric ceramics layers are extended or led to different side surfaces opposite to each other and connected to input electrodes formed on the side surfaces of the layered structure, respectively. On the other hand, the output portion comprises strip-like electrodes arranged in parallel in the longitudinal direction of the layered structure and alternately stacked with the piezoelectric ceramics layers. Both sides of these inner electrodes are exposed at the opposite side surfaces and connected to a pair of output electrodes, respectively, which are formed on the opposite side surfaces, respectively, and is opposite to each other. The input electrodes are provided with lead wires connected to an input side. The output electrodes are provided with lead wires connected to an output side. One of the lead wires of the output portion has a high potential while the adjacent lead wires have a low and a same potential.
However, a widened range of luminance control has been required to be processed for an inverter utilizing the piezoelectric transformer, which may often adopts burst adjustment of brightness increasingly so as to meet the requirement.
This results in a problem of generation of an audible sound, which has not occurred in a conventional mounting method. The reason is as follows. The vibration of the piezoelectric transformer has traditionally been excited in a single vibration mode in most cases. However, by addition of the brightness adjusting function, the vibration of the piezoelectric transformer is no longer a single mode vibration and thereby affects electrical junctions thereof.
In addition to the problem that the vibration mode is no longer a single mode vibration, there is another problem due to transition into a small and thin device of the piezoelectric transformer element that an area is limited for electrical connection upon an electrode structure thereof, thereby resulting in a problem in reliability of electrical connection between the piezoelectric transformer element and the circuit board.
Therefore, it is a primary object of the present invention to provide a piezoelectric transformer which can minimize the number of steps and the time required for mounting, as compared with a conventional mounting method.
It is a second object of the present invention to provide a mounting method of the above-mentioned piezoelectric transformer.
It is a third object of the present invention to provide a piezoelectric transformer which can reduce a sound pressure level of an audible sound without deteriorating electric characteristics such as a step-up ratio and heat generation, and can improve reliability.
It is a fourth object of the present invention to provide a piezoelectric transformer which can realize a very thin piezoelectric inverter.
It is a fourth object of the present invention to provide a power supply has a piezoelectric inverter utilizing the above-mentioned piezoelectric transformer.